shared: add PTY helper

This Pty API wraps the ugliness that is POSIX PTY. It takes care of:
  - edge-triggered HUP handling (avoid heavy CPU-usage on vhangup)
  - HUP vs. input-queue draining (handle HUP _after_ draining the whole
    input queue)
  - SIGCHLD vs. HUP (HUP is no reliable way to catch PTY deaths, always
    use SIGCHLD. Otherwise, vhangup() and friends will break.)
  - Output queue buffering (async EPOLLOUT handling)
  - synchronous setup (via Barrier API)

At the same time, the PTY API does not execve(). It simply fork()s and
leaves everything else to the caller. Usually, they execve() but we
support other setups, too.

This will be needed by multiple UI binaries (systemd-console, systemd-er,
...) so it's placed in src/shared/. It's not strictly related to
libsystemd-terminal, so it's not included there.

2 files changed, 717 insertions, 0 deletions

diff --git a/src/shared/pty.c b/src/shared/pty.c
new file mode 100644
index 0000000000..11d76f825f
--- /dev/null
+++ b/src/shared/pty.c
@@ -0,0 +1,640 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 David Herrmann <dh.herrmann@gmail.com>
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+/*
+ * PTY
+ * A PTY object represents a single PTY connection between a master and a
+ * child. The child process is fork()ed so the caller controls what program
+ * will be run.
+ *
+ * Programs like /bin/login tend to perform a vhangup() on their TTY
+ * before running the login procedure. This also causes the pty master
+ * to get a EPOLLHUP event as long as no client has the TTY opened.
+ * This means, we cannot use the TTY connection as reliable way to track
+ * the client. Instead, we _must_ rely on the PID of the client to track
+ * them.
+ * However, this has the side effect that if the client forks and the
+ * parent exits, we loose them and restart the client. But this seems to
+ * be the expected behavior so we implement it here.
+ *
+ * Unfortunately, epoll always polls for EPOLLHUP so as long as the
+ * vhangup() is ongoing, we will _always_ get EPOLLHUP and cannot sleep.
+ * This gets worse if the client closes the TTY but doesn't exit.
+ * Therefore, the fd must be edge-triggered in the epoll-set so we
+ * only get the events once they change.
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <pty.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/epoll.h>
+#include <sys/eventfd.h>
+#include <sys/ioctl.h>
+#include <sys/types.h>
+#include <sys/uio.h>
+#include <sys/wait.h>
+#include <termios.h>
+#include <unistd.h>
+
+#include "barrier.h"
+#include "macro.h"
+#include "pty.h"
+#include "ring.h"
+#include "util.h"
+
+#define PTY_BUFSIZE 16384
+
+enum {
+        PTY_ROLE_UNKNOWN,
+        PTY_ROLE_PARENT,
+        PTY_ROLE_CHILD,
+};
+
+struct Pty {
+        unsigned long ref;
+        Barrier barrier;
+        int fd;
+        pid_t child;
+        sd_event_source *fd_source;
+        sd_event_source *child_source;
+
+        char in_buf[PTY_BUFSIZE];
+        Ring out_buf;
+
+        pty_event_t event_fn;
+        void *event_fn_userdata;
+
+        bool needs_requeue : 1;
+        unsigned int role : 2;
+};
+
+int pty_new(Pty **out) {
+        _pty_unref_ Pty *pty = NULL;
+        int r;
+
+        assert_return(out, -EINVAL);
+
+        pty = new0(Pty, 1);
+        if (!pty)
+                return -ENOMEM;
+
+        pty->ref = 1;
+        pty->fd = -1;
+
+        pty->fd = posix_openpt(O_RDWR | O_NOCTTY | O_CLOEXEC | O_NONBLOCK);
+        if (pty->fd < 0)
+                return -errno;
+
+        /*
+         * The slave-node is initialized to uid/gid of the caller of
+         * posix_openpt(). Only if devpts is mounted with fixed uid/gid this is
+         * skipped. In that case, grantpt() can overwrite these, but then you
+         * have to be root to use chown() (or a pt_chown helper has to be
+         * present). In those cases grantpt() really does something,
+         * otherwise it's a no-op. We call grantpt() here to try supporting
+         * those cases, even though no-one uses that, I guess. If you need other
+         * access-rights, set them yourself after this call returns (no, this is
+         * not racy, it looks racy, but races regarding your own UID are never
+         * important as an attacker could ptrace you; and the slave-pty is also
+         * still locked).
+         */
+        r = grantpt(pty->fd);
+        if (r < 0)
+                return -errno;
+
+        r = barrier_init(&pty->barrier);
+        if (r < 0)
+                return r;
+
+        *out = pty;
+        pty = NULL;
+        return 0;
+}
+
+Pty *pty_ref(Pty *pty) {
+        if (!pty || pty->ref < 1)
+                return NULL;
+
+        ++pty->ref;
+        return pty;
+}
+
+Pty *pty_unref(Pty *pty) {
+        if (!pty || pty->ref < 1 || --pty->ref > 0)
+                return NULL;
+
+        pty_close(pty);
+        pty->child_source = sd_event_source_unref(pty->child_source);
+        barrier_destroy(&pty->barrier);
+        ring_clear(&pty->out_buf);
+        free(pty);
+
+        return NULL;
+}
+
+Barrier *pty_get_barrier(Pty *pty) {
+        assert(pty);
+        return &pty->barrier;
+}
+
+bool pty_is_unknown(Pty *pty) {
+        return pty && pty->role == PTY_ROLE_UNKNOWN;
+}
+
+bool pty_is_parent(Pty *pty) {
+        return pty && pty->role == PTY_ROLE_PARENT;
+}
+
+bool pty_is_child(Pty *pty) {
+        return pty && pty->role == PTY_ROLE_CHILD;
+}
+
+bool pty_has_child(Pty *pty) {
+        return pty_is_parent(pty) && pty->child > 0;
+}
+
+pid_t pty_get_child(Pty *pty) {
+        return pty_has_child(pty) ? pty->child : -ECHILD;
+}
+
+bool pty_is_open(Pty *pty) {
+        return pty && pty->fd >= 0;
+}
+
+int pty_get_fd(Pty *pty) {
+        assert_return(pty, -EINVAL);
+
+        return pty_is_open(pty) ? pty->fd : -EPIPE;
+}
+
+int pty_make_child(Pty *pty) {
+        char slave_name[1024];
+        int r, fd;
+
+        assert_return(pty, -EINVAL);
+        assert_return(pty_is_unknown(pty), -EALREADY);
+
+        r = ptsname_r(pty->fd, slave_name, sizeof(slave_name));
+        if (r < 0)
+                return -errno;
+
+        fd = open(slave_name, O_RDWR | O_CLOEXEC | O_NOCTTY);
+        if (fd < 0)
+                return -errno;
+
+        safe_close(pty->fd);
+        pty->fd = fd;
+        pty->child = getpid();
+        pty->role = PTY_ROLE_CHILD;
+        barrier_set_role(&pty->barrier, BARRIER_CHILD);
+
+        return 0;
+}
+
+int pty_make_parent(Pty *pty, pid_t child) {
+        assert_return(pty, -EINVAL);
+        assert_return(pty_is_unknown(pty), -EALREADY);
+
+        pty->child = child;
+        pty->role = PTY_ROLE_PARENT;
+
+        return 0;
+}
+
+int pty_unlock(Pty *pty) {
+        assert_return(pty, -EINVAL);
+        assert_return(pty_is_unknown(pty) || pty_is_parent(pty), -EINVAL);
+        assert_return(pty_is_open(pty), -ENODEV);
+
+        return unlockpt(pty->fd) < 0 ? -errno : 0;
+}
+
+int pty_setup_child(Pty *pty) {
+        struct termios attr;
+        pid_t pid;
+        int r;
+
+        assert_return(pty, -EINVAL);
+        assert_return(pty_is_child(pty), -EINVAL);
+        assert_return(pty_is_open(pty), -EALREADY);
+
+        r = sigprocmask_many(SIG_SETMASK, -1);
+        if (r < 0)
+                return r;
+
+        r = reset_all_signal_handlers();
+        if (r < 0)
+                return r;
+
+        pid = setsid();
+        if (pid < 0 && errno != EPERM)
+                return -errno;
+
+        r = ioctl(pty->fd, TIOCSCTTY, 0);
+        if (r < 0)
+                return -errno;
+
+        r = tcgetattr(pty->fd, &attr);
+        if (r < 0)
+                return -errno;
+
+        /* erase character should be normal backspace, PLEASEEE! */
+        attr.c_cc[VERASE] = 010;
+        /* always set UTF8 flag */
+        attr.c_iflag |= IUTF8;
+
+        r = tcsetattr(pty->fd, TCSANOW, &attr);
+        if (r < 0)
+                return -errno;
+
+        if (dup2(pty->fd, STDIN_FILENO) != STDIN_FILENO ||
+            dup2(pty->fd, STDOUT_FILENO) != STDOUT_FILENO ||
+            dup2(pty->fd, STDERR_FILENO) != STDERR_FILENO)
+                return -errno;
+
+        /* only close FD if it's not a std-fd */
+        pty->fd = (pty->fd > 2) ? safe_close(pty->fd) : -1;
+
+        return 0;
+}
+
+void pty_close(Pty *pty) {
+        if (!pty_is_open(pty))
+                return;
+
+        pty->fd_source = sd_event_source_unref(pty->fd_source);
+        pty->fd = safe_close(pty->fd);
+}
+
+/*
+ * Drain input-queue and dispatch data via the event-handler. Returns <0 on
+ * error, 0 if queue is empty and 1 if we couldn't empty the input queue fast
+ * enough and there's still data left.
+ */
+static int pty_dispatch_read(Pty *pty) {
+        unsigned int i;
+        ssize_t len;
+        int r;
+
+        /*
+         * We're edge-triggered, means we need to read the whole queue. This,
+         * however, might cause us to stall if the writer is faster than we
+         * are. Therefore, we read twice and if the second read still returned
+         * data, we reschedule.
+         */
+
+        for (i = 0; i < 2; ++i) {
+                len = read(pty->fd, pty->in_buf, sizeof(pty->in_buf) - 1);
+                if (len < 0) {
+                        if (errno == EINTR)
+                                continue;
+
+                        return (errno == EAGAIN) ? 0 : -errno;
+                } else if (len == 0) {
+                        continue;
+                }
+
+                /* set terminating zero for debugging safety */
+                pty->in_buf[len] = 0;
+                r = pty->event_fn(pty, pty->event_fn_userdata, PTY_DATA, pty->in_buf, len);
+                if (r < 0)
+                        return r;
+        }
+
+        /* still data left, make sure we're queued again */
+        pty->needs_requeue = true;
+
+        return 1;
+}
+
+/*
+ * Drain output-queue by writing data to the pty. Returns <0 on error, 0 if the
+ * output queue is empty now and 1 if we couldn't empty the output queue fast
+ * enough and there's still data left.
+ */
+static int pty_dispatch_write(Pty *pty) {
+        struct iovec vec[2];
+        unsigned int i;
+        ssize_t len;
+        size_t num;
+
+        /*
+         * Same as pty_dispatch_read(), we're edge-triggered so we need to call
+         * write() until either all data is written or it returns EAGAIN. We
+         * call it twice and if it still writes successfully, we reschedule.
+         */
+
+        for (i = 0; i < 2; ++i) {
+                num = ring_peek(&pty->out_buf, vec);
+                if (num < 1)
+                        return 0;
+
+                len = writev(pty->fd, vec, (int)num);
+                if (len < 0) {
+                        if (errno == EINTR)
+                                continue;
+
+                        return (errno == EAGAIN) ? 1 : -errno;
+                } else if (len == 0) {
+                        continue;
+                }
+
+                ring_pull(&pty->out_buf, (size_t)len);
+        }
+
+        /* still data left, make sure we're queued again */
+        if (ring_get_size(&pty->out_buf) > 0) {
+                pty->needs_requeue = true;
+                return 1;
+        }
+
+        return 0;
+}
+
+static int pty_fd_fn(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
+        Pty *pty = userdata;
+        int r_hup = 0, r_write = 0, r_read = 0, r;
+
+        /*
+         * Whenever we encounter I/O errors, we have to make sure to drain the
+         * input queue first, before we handle any HUP. A child might send us
+         * a message and immediately close the queue. We must not handle the
+         * HUP first or we loose data.
+         * Therefore, if we read a message successfully, we always return
+         * success and wait for the next event-loop iteration. Furthermore,
+         * whenever there is a write-error, we must try reading from the input
+         * queue even if EPOLLIN is not set. The input might have arrived in
+         * between epoll_wait() and write(). Therefore, write-errors are only
+         * ever handled if the input-queue is empty. In all other cases they
+         * are ignored until either reading fails or the input queue is empty.
+         */
+
+        if (revents & (EPOLLHUP | EPOLLERR))
+                r_hup = -EPIPE;
+
+        if (revents & EPOLLOUT)
+                r_write = pty_dispatch_write(pty);
+
+        /* Awesome! Kernel signals HUP without IN but queues are not empty.. */
+        if ((revents & EPOLLIN) || r_hup < 0 || r_write < 0) {
+                r_read = pty_dispatch_read(pty);
+                if (r_read > 0)
+                        return 0; /* still data left to fetch next round */
+        }
+
+        if (r_hup < 0 || r_write < 0 || r_read < 0) {
+                /* PTY closed and input-queue drained */
+                pty_close(pty);
+                r = pty->event_fn(pty, pty->event_fn_userdata, PTY_HUP, NULL, 0);
+                if (r < 0)
+                        return r;
+        }
+
+        return 0;
+}
+
+static int pty_fd_prepare_fn(sd_event_source *source, void *userdata) {
+        Pty *pty = userdata;
+        int r;
+
+        if (pty->needs_requeue) {
+                /*
+                 * We're edge-triggered. In case we couldn't handle all events
+                 * or in case new write-data is queued, we set needs_requeue.
+                 * Before going asleep, we set the io-events *again*. sd-event
+                 * notices that we're edge-triggered and forwards the call to
+                 * the kernel even if the events didn't change. The kernel will
+                 * check the events and re-queue us on the ready queue in case
+                 * an event is pending.
+                 */
+                r = sd_event_source_set_io_events(source, EPOLLHUP | EPOLLERR | EPOLLIN | EPOLLOUT | EPOLLET);
+                if (r >= 0)
+                        pty->needs_requeue = false;
+        }
+
+        return 0;
+}
+
+static int pty_child_fn(sd_event_source *source, const siginfo_t *si, void *userdata) {
+        Pty *pty = userdata;
+        int r;
+
+        pty->child = 0;
+
+        r = pty->event_fn(pty, pty->event_fn_userdata, PTY_CHILD, si, sizeof(*si));
+        if (r < 0)
+                return r;
+
+        return 0;
+}
+
+int pty_attach_event(Pty *pty, sd_event *event, pty_event_t event_fn, void *event_fn_userdata) {
+        int r;
+
+        assert_return(pty, -EINVAL);
+        assert_return(event, -EINVAL);
+        assert_return(event_fn, -EINVAL);
+        assert_return(pty_is_parent(pty), -EINVAL);
+
+        pty_detach_event(pty);
+
+        if (pty_is_open(pty)) {
+                r = sd_event_add_io(event,
+                                    &pty->fd_source,
+                                    pty->fd,
+                                    EPOLLHUP | EPOLLERR | EPOLLIN | EPOLLOUT | EPOLLET,
+                                    pty_fd_fn,
+                                    pty);
+                if (r < 0)
+                        goto error;
+
+                r = sd_event_source_set_prepare(pty->fd_source, pty_fd_prepare_fn);
+                if (r < 0)
+                        goto error;
+        }
+
+        if (pty_has_child(pty)) {
+                r = sd_event_add_child(event,
+                                       &pty->child_source,
+                                       pty->child,
+                                       WEXITED,
+                                       pty_child_fn,
+                                       pty);
+                if (r < 0)
+                        goto error;
+        }
+
+        pty->event_fn = event_fn;
+        pty->event_fn_userdata = event_fn_userdata;
+
+        return 0;
+
+error:
+        pty_detach_event(pty);
+        return r;
+}
+
+void pty_detach_event(Pty *pty) {
+        if (!pty)
+                return;
+
+        pty->child_source = sd_event_source_unref(pty->child_source);
+        pty->fd_source = sd_event_source_unref(pty->fd_source);
+        pty->event_fn = NULL;
+        pty->event_fn_userdata = NULL;
+}
+
+int pty_write(Pty *pty, const void *buf, size_t size) {
+        bool was_empty;
+        int r;
+
+        assert_return(pty, -EINVAL);
+        assert_return(pty_is_open(pty), -ENODEV);
+        assert_return(pty_is_parent(pty), -ENODEV);
+
+        if (size < 1)
+                return 0;
+
+        /*
+         * Push @buf[0..@size] into the output ring-buffer. In case the
+         * ring-buffer wasn't empty beforehand, we're already waiting for
+         * EPOLLOUT and we're done. If it was empty, we have to re-queue the
+         * FD for EPOLLOUT as we're edge-triggered and wouldn't get any new
+         * EPOLLOUT event.
+         */
+
+        was_empty = ring_get_size(&pty->out_buf) < 1;
+
+        r = ring_push(&pty->out_buf, buf, size);
+        if (r < 0)
+                return r;
+
+        if (was_empty)
+                pty->needs_requeue = true;
+
+        return 0;
+}
+
+int pty_signal(Pty *pty, int sig) {
+        assert_return(pty, -EINVAL);
+        assert_return(pty_is_open(pty), -ENODEV);
+        assert_return(pty_is_parent(pty), -ENODEV);
+
+        return ioctl(pty->fd, TIOCSIG, sig) < 0 ? -errno : 0;
+}
+
+int pty_resize(Pty *pty, unsigned short term_width, unsigned short term_height) {
+        struct winsize ws;
+
+        assert_return(pty, -EINVAL);
+        assert_return(pty_is_open(pty), -ENODEV);
+        assert_return(pty_is_parent(pty), -ENODEV);
+
+        zero(ws);
+        ws.ws_col = term_width;
+        ws.ws_row = term_height;
+
+        /*
+         * This will send SIGWINCH to the pty slave foreground process group.
+         * We will also get one, but we don't need it.
+         */
+        return ioctl(pty->fd, TIOCSWINSZ, &ws) < 0 ? -errno : 0;
+}
+
+pid_t pty_fork(Pty **out, sd_event *event, pty_event_t event_fn, void *event_fn_userdata, unsigned short initial_term_width, unsigned short initial_term_height) {
+        _pty_unref_ Pty *pty = NULL;
+        int r;
+        pid_t pid;
+
+        assert_return(out, -EINVAL);
+        assert_return((event && event_fn) || (!event && !event_fn), -EINVAL);
+
+        r = pty_new(&pty);
+        if (r < 0)
+                return r;
+
+        r = pty_unlock(pty);
+        if (r < 0)
+                return r;
+
+        pid = fork();
+        if (pid < 0)
+                return -errno;
+
+        if (pid == 0) {
+                /* child */
+
+                r = pty_make_child(pty);
+                if (r < 0)
+                        _exit(-r);
+
+                r = pty_setup_child(pty);
+                if (r < 0)
+                        _exit(-r);
+
+                /* sync with parent */
+                if (!barrier_place_and_sync(&pty->barrier))
+                        _exit(1);
+
+                /* fallthrough and return the child's PTY object */
+        } else {
+                /* parent */
+
+                r = pty_make_parent(pty, pid);
+                if (r < 0)
+                        goto parent_error;
+
+                r = pty_resize(pty, initial_term_width, initial_term_height);
+                if (r < 0)
+                        goto parent_error;
+
+                if (event) {
+                        r = pty_attach_event(pty, event, event_fn, event_fn_userdata);
+                        if (r < 0)
+                                goto parent_error;
+                }
+
+                /* sync with child */
+                if (!barrier_place_and_sync(&pty->barrier)) {
+                        r = -ECHILD;
+                        goto parent_error;
+                }
+
+                /* fallthrough and return the parent's PTY object */
+        }
+
+        *out = pty;
+        pty = NULL;
+        return pid;
+
+parent_error:
+        barrier_abort(&pty->barrier);
+        waitpid(pty->child, NULL, 0);
+        pty->child = 0;
+        return r;
+}
diff --git a/src/shared/pty.h b/src/shared/pty.h
new file mode 100644
index 0000000000..a87ceb58ca
--- /dev/null
+++ b/src/shared/pty.h
@@ -0,0 +1,77 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 David Herrmann <dh.herrmann@gmail.com>
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <errno.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "barrier.h"
+#include "macro.h"
+#include "sd-event.h"
+#include "util.h"
+
+typedef struct Pty Pty;
+
+enum {
+        PTY_CHILD,
+        PTY_HUP,
+        PTY_DATA,
+};
+
+typedef int (*pty_event_t) (Pty *pty, void *userdata, unsigned int event, const void *ptr, size_t size);
+
+int pty_new(Pty **out);
+Pty *pty_ref(Pty *pty);
+Pty *pty_unref(Pty *pty);
+
+#define _pty_unref_ _cleanup_(pty_unrefp)
+DEFINE_TRIVIAL_CLEANUP_FUNC(Pty*, pty_unref);
+
+Barrier *pty_get_barrier(Pty *pty);
+
+bool pty_is_unknown(Pty *pty);
+bool pty_is_parent(Pty *pty);
+bool pty_is_child(Pty *pty);
+bool pty_has_child(Pty *pty);
+pid_t pty_get_child(Pty *pty);
+
+bool pty_is_open(Pty *pty);
+int pty_get_fd(Pty *pty);
+
+int pty_make_child(Pty *pty);
+int pty_make_parent(Pty *pty, pid_t child);
+int pty_unlock(Pty *pty);
+int pty_setup_child(Pty *pty);
+void pty_close(Pty *pty);
+
+int pty_attach_event(Pty *pty, sd_event *event, pty_event_t event_fn, void *event_fn_userdata);
+void pty_detach_event(Pty *pty);
+
+int pty_write(Pty *pty, const void *buf, size_t size);
+int pty_signal(Pty *pty, int sig);
+int pty_resize(Pty *pty, unsigned short term_width, unsigned short term_height);
+
+pid_t pty_fork(Pty **out, sd_event *event, pty_event_t event_fn, void *event_fn_userdata, unsigned short initial_term_width, unsigned short initial_term_height);